Why Does My Mac Menu Bar Disappear While I Am In Safari?

It sounds like you have Safari running in Full Screen mode. Try moving your cursor to the top of the display where the menubar should be and hold it there for about a second. If the menubar drops down, Safari was indeed in Full Screen mode. To the far right you shold see a blue box with arrows in it. Click on the box to exit Full Screen mode.

This does not help me. It is worse in "not full screen" as I have nowhere to type the url.

Thanks anyone who might help

I had a problem in Yosemite where the top menu just disappeared, so I could not even tell if I was in Firefox or Safari. I could not see the toolbar which contained, for example, "Safari File Edit View History Bookmarks Develop Window Help". The same apple for the Firefox, Excel and all the other top toolbars.

I accidentally solved it when I was in Excel and pressed option-command-v. This locked up and destroyed my Excel Spreadsheet and there was no "Undo" option on that command either. The side effect was that the top toolbar came back. It was a fault I have never seen before and one which I could not easily find on ant websites. Your fault seems to be one of the closest to what I had.

That used to work. But now the blue box doesn't appear.